This invention relates to an automatic system for analyzing specimens. Cross reference is made to three related copending applications assigned to the same assignee and filed in the name of Richard W. Hanaway entitled "Tower for Analyzing System", now U.S. Pat. No. 4,643,879; entitled "Tray for Analyzing System", U.S. application Ser. No. 06/750,794 filed July 1, 1985; and entitled "Reagent Dispenser for Analyzing System", U.S. application Ser. No. 06/750,791, filed July, 1, 1985, now abandoned.
This invention relates to an automatic specimen analyzing system which substantially reduces operator involvement over presently available systems. After the operator loads the specimen trays into the system of this invention, various operations including incubation after inoculation, adding reagents and analysis of the specimen following incubation are all handled automatically without further operator involvement. A computer-type processor controls the system so that the various operations are carried out in appropriate sequence and the results of the analysis are recorded with specific reference to the sample analyzed.
Automation in microbiology has lagged far behind chemistry and hematology in the clinical laboratory. However, there is presently an intensive effort by industry to develop this field. The best publicized devices for performing automated antimicrobic susceptibility testing use optical detection methods. A continuous flow device for detecting particles 0.5 micron or less has been commercially available since 1970; however, probably due to its great expense, it has not been widely used in the laboratory. Other devices using laser light sources have been suggested but have not proven commercially practicable. Recently, the most attention has been directed to three devices discussed below.
The Pfizer Autobac 1 system (U.S. Pat. No. Re. 28,801) measures relative bacterial growth by light scatter at a fixed 35 degree angle. It includes twelve test chambers and one control chamber in a plastic device that forms multiple contiguous cuvettes. Antibiotics are introduced to the chambers via impregnated paper discs. The antimicrobic sensitivity reader comes with an incubator, shaker, and disc dispenser. Results are expressed as a light scattering index (LSI), and these numbers are related to the Kirby-Bauer "sensitive, intermediate and resistant." MIC measurements which are not available routinely with this instrument. In a comparison with susceptibilities of clinical isolates measured by the Kirby-Bauer method, there was 91% agreement. However, with this system some bacteria strain-drug combinations have been found to produce a resistant Kirby-Bauer zone diameter and at the same time a sensitive LSI.
The Auto Microbic System has been developed by McDonnell-Douglas to perform identification, enumeration and susceptibility studies on nine urinary tract pathogens using a plastic plate containing a 4.times.5 array of wells. (See Gibson et al. U.S. Pat. No. 3,957,583; Charles et al, U.S. Pat. No. 4,118,280, and Charles et al, U.S. Pat. No. 4,116,775.) The specimen is drawn into the small wells by negative pressure and the instrument monitors the change in optical absorbance and scatter with light-emitting diodes and an array of optical sensors. A mechanical device moves each plate into a sensing slot in a continuous succession so that each plate is scanned at the rate of one an hour, and an onboard digital computer stores the optical data. The system will process either 120 or 240 specimens at a time. One can query the status of each test via a CRT-keyboard console, and hard copy can be made from any display. When the system detects sufficient bacterial growth to permit a valid result, in automatically triggers a print-out. Following identification in four to thirteen hours, a technologist transfers positive cultures to another system which tests for antimicrobic susceptibility. The results are expressed as "R" (resistant) and "S" (susceptible); however, no quantitative MIC data are provided.
It should be noted that Gibson et al, U.S. Pat. No. 3,957,583 do not include automation techniques, but use naked-eye inspection or a manually-operated colorimeter. Scanning is therefore a hand or a mechanical operation. Charles et al, U.S. Pat. Nos. 4,116,775 and 4,118,280 also require mechanical movement of their cassette for reading different rows.
The Abbot MS-2 system consists of chambers composed of eleven contiguous cuvettes. Similar to the Pfizer Autobac 1, the antimicrobial compounds are introduced by way of impregnated paper discs. An inoculum consisting of a suspension of organisms from several colonies is introduced into the culture medium, and the cuvette cartridge is filled with this suspension. The operator inserts the cuvette cartridge into an analysis module which will handle eight cartridges (additional modules can be added to the system). Following agitation of the cartridge, the instrument monitors the growth rate by turbidimetry. When the log growth phase occurs, the system automatically transfers the broth solution to the eleven cuvette chambers; ten of these chambers contain antimicrobial discs while the eleventh is a growth control.
The device performs readings at five minute intervals, and stores the data in a microprocessor. Following a pre-set increase of turbidity of the growth control, the processor establishes a growth rate constant for each chamber. A comparison of the antimicrobic growth rate constant and control growth rate constant forms the basis of susceptibility calculations. The printout presents results as either resistant or susceptible and if intermediate, susceptibility information is expressed as an MIC.
Non-optical methods have also been used or suggested for measuring antimicrobic sensitivity in susceptibility testing. These have included radiorespirometry, electrical impedance, bioluminescence and microcalorimetry. Radiorespirometry, based on the principle that bacteria metabolized carbohydrate and the carbohydrate carbon may be detected following its release as CO.sub.2 involves the incorporation of the isotope C14 into carbohydrates. Released C.sup.14 O.sub.2 gas is trapped and beta counting techniques are used to detect the isotope.
The major difficulty in applying the isotope detection system to susceptibility testing, however, is that an antimicrobic agent may be able to stop growth of a species of bacteria, yet metabolism of carbohydrate may continue. Less likely, a given drug may turn off the metabolic machinery that metabolizes certain carbohydrates, but growth may continue. This dissociation between metabolism and cell growth emphasizes the fact that measurments for detecting antimicrobic susceptibility should depend upon a determination of cell mass or cell number rather than metabolism.
The electrical impedance system is based on the fact that bacterial cells have a low net charge and higher electrical impedance than the surrounding electrolytic bacterial growth media. A pulse impedance cell-counting device can be used to count the cells; however, available counting devices are not designed to handle batches of samples automatically, and generally do not have the capacity to distinguish between live and dead bacterial cells.
Another approach with electrical impedance has been to monitor the change in the conductivity of the media during the growth phase of bacteria. As bacteria utilize the nutrients, they produce metabolites which have a greater degree of electrical conductance than the native broth so that as metabolism occurs, impedance decreases. However, since this technique measures cell metabolism rather than cell mass, its applicability to antimicrobic susceptibility detection suffers from the same drawback as radiorespirometry.
Bioluminescence has also been suggested for the detection of microorganisms. It is based on the principle that a nearly universal property of living organisms is the storage of energy in the form of high energy phosphates (adenosine triphosphate, ATP), which can be detected through reaction with firefly luciferase. The reaction results in the emission of light energy which can be detected with great sensitivity by electronic light transducers. Although a clinical laboratory may obtain a bioluminescence system to detect the presence of bacteria in urine, the technique is expensive due to the limited availability of firefly luciferase, and problems have been encountered in standardizing the system.
Microcalorimetry is the measurement of minute amounts of heat generated by bacterial metabolism. The principle exhibits certain advantages, but laboratories have not adopted such a system, one serious drawback being that the system measures metabolic activity rather than bacterial mass or number.
In U.S. application Ser. No. 082,228, filed on Oct. 5, 1979, by Wertz, Hathaway and Cook, now U.S. Pat. No. 4,448,534, granted May 15, 1984, assigned to the assignee of the present invention, an automatic scanning apparatus for performing optical density tests on liquid samples as well as methods for testing for antibiotic susceptibility and identifying microorganisms is disclosed. The apparatus of the prior application includes a system for automatically scanning electronically each well of a multi-well tray containing many liquid samples. A light source, preferably a single source, is passed through the wells to an array of photosensitive cells, one for each well. There is also a calibrating or comparison cell receiving the light. Electronic apparatus read each cell in sequence quickly completing the scan without physical movement of any parts. The resultant signals are compared with the signals from a comparison cell and with other signals or stored data, and determinations are made and displayed or printed out.
A system of the type described in this prior application is sold under the trademarks "MicroScan" and "autoSCAN-3" by the American Scientific Products Division of Americal Hospital Supply Corporation, McGraw Park, Ill.
A description of the MicroScan System appears in a brochure covering it which was published in 1981.
While the MicroScan System represents a substantial advancement in the automation of microbiological analysis, it still requires operator involvement to handle operations such as incubation, addition of reagents and insertion for the autoscan analysis operation. In other words, for the MicroScan System, presently in use, an operator must perform the operations of placing the tray in a suitable system for incubation for the desired period and after incubation, adding reagents and inserting the tray in the analyzer. In accordance with the present invention, all of these operations after insertion of the tray in the system are carried out fully and automatically.